Audio recording and playback systems can be characterized by the number of individual channel or tracks used to input and/or play back a group of sounds. In a basic stereo recording system, two channels each connected to a microphone may be used to record sounds detected from the distinct microphone locations. Upon playback, the sounds recording by the two channels are typically reproduced through a pair of loudspeakers, with one loudspeaker reproducing an individual channel. Providing two separate audio channels for recording permits individual processing of these channels to achieve an intended effect upon playback. Similarly, providing more discrete audio channels allows more freedom in isolating certain sounds to enable the separate processing of these sounds.
Professional audio studios use multiple channel recordings systems which can isolate and process numerous individual sounds. However, since many conventional audio reproduction devices are delivered in traditional stereo, use of a multi-channel system to record sounds requires that the sounds be "mixed" down to only two individual signals. In the professional audio recording world, studios employ such mixing methods since individual instruments and vocals of a given audio work may be initially recorded on separate tracks, but must be replayed in a stereo format found in conventional stereo systems. Professional systems may use 48 or more separate audio channels which are processed individually before recorded onto two stereo tracks.
In multi-channel playback systems, i.e., defined herein as systems having more than two individual audio channels, each sound recorded from an individual channel may be separately processed and played through a corresponding speaker or speakers. Thus, sounds which are recorded from, or intended to be placed at, multiple locations about a listener, can be realistically reproduced through a dedicated speaker placed at the appropriate location. Such systems have found particular use in theaters and other audio-visual environments where a captive and fixed audience experiences both an audio and visual presentation. These systems, which include Dolby Laboratories' "Dolby Digital" system; the Digital Theater System (DTS); and Sony's Dynamic Digital Sound (SDDS), are all designed to initially record and then reproduce multi-channel sounds to provide a surround listening experience.
In the personal computer and home theater arena, recorded media is being standardized so that multiple channels, in addition to the two conventional stereo channels, are stored on such recorded media. One such standard is Dolby's AC-3 multi-channel encoding standard which provides six separate audio signals. In the Dolby AC-3 system, two audio channels are intended for playback on forward left and right speakers, two channels are reproduced on rear left and right speakers, one channel is used for a forward center dialogue speaker, and one channel is used for low-frequency and effects signals. Audio playback systems which can accommodate the reproduction of all these six channels do not require that the signals be mixed into a two channel format. However, many playback systems, including today's typical personal computer and tomorrow's personal computer/television, may have only two channel playback capability (excluding center and subwoofer channels). Accordingly, the information present in additional audio signals, apart from that of the conventional stereo signals, like those found in an AC-3 recording, must either be electronically discarded or mixed into a two channel format.
There are various techniques and methods for mixing multi-channel signals into a two channel format. A simple mixing method may be to simply combine all of the signals into a two-channel format while adjusting only the relative gains of the mixed signals. Other techniques may apply frequency shaping, amplitude adjustments, time delays or phase shifts, or some combination of all of these, to an individual audio signal during the final mixing process. The particular technique or techniques used may depend on the format and content of the individual audio signals as well as the intended use of the final two channel mix.
For example, U.S. Pat. No. 4,393,270 issued to van den Berg discloses a method of processing electrical signals by modulating each individual signal corresponding to a preselected direction of perception which may compensate for placement of a loudspeaker. A separate multi-channel processing system is disclosed in U.S. Pat. No. 5,438,623 issued to Begault. In Begault, individual audio signals are divided into two signals which are each delayed and filtered according to a head related transfer function (HRTF) for the left and right ears. The resultant signals are then combined to generate left and right output signals intended for playback through a set of headphones.
The techniques found in the prior art, including those found in the professional recording arena, do not provide an effective method for mixing multi-channel signals into a two channel format to achieve a realistic audio reproduction through a limited number of discrete channels. As a result, much of the ambiance information which provides an immersive sense of sound perception may be lost or masked in the final mixed recording. Despite numerous previous methods of processing multi-channel audio signals to achieve a realistic experience through conventional two channel playback, there is much room for improvement to achieve the goal of a realistic listening experience.
Accordingly, it is an object of the present invention to provide an improved method of mixing multi-channel audio signals which can be used in all aspects of recording and playback to provide an improved and realistic listening experience. It is an object of the present invention to provide an improved system and method for mastering professional audio recordings intended for playback on a conventional stereo system. It is also an object of the present invention to provide a system and method to process multi-channel audio signals extracted from an audio-visual recording to provide an immersive listening experience when reproduced through a limited number of audio channels.
For example, personal computers and video players are emerging with the capability to record and reproduce digital video disks (DVD) having six or more discrete audio channels. However, since many such computers and video players do not have more than two audio playback channels (and possibly one sub-woofer channel), they cannot use the full amount of discrete audio channels as intended in a surround environment. Thus, there is a need in the art for a computer and other video delivery system which can effectively use all of the audio information available in such systems and provide a two channel listening experience which rivals multi-channel playback systems. The present invention fulfills this need.